Objectives: To characterize: a) bacterial lipopolysaccharide (LPS) as a helper factor for the expression of a leukemia virus in genetically resistant mice; b) the mechanism of action of the Fv-2r resistance gene in mice; c) the nature of murine leukemia virus host range conversion from N-to NB-tropism. Approach: Inbred mice are injected intravenously with Friend virus (FV) complex and killed 9 days later. The numbers of macroscopically visible foci on the surfaces of their spleens fixed in Bouin's fluid are directly proportional to the exression of a defective virus in FV complex, called spleen focus-forming virus (SFFV). The addition of LPS to the virus inoculum greatly increases the expression of SFFV in genetically resistant mice, but gives less "helper" activity if injected into mice before or after virus infection. Tests will be conducted on SFFV expression in mice treated to suppress or enhance certain physiological types of cells (e.g., T-lymphocytes), in mice lacking endogenous mouse-tropic leukemia viruses, and in mice congenic for certain resistance genes (Fv-1b and Fv-2r).